1. Field of the Invention
This invention relates to a process for producing methacrylic acid esters. More particularly, it is concerned with a continuous process for production of methacrylic acid esters in the presence of sulfuric acid as a catalyst in at least two reaction zones connected in series.
2. Description of the Prior Art
Various methods have been proposed to produce methacrylic acid esters from methacrylic acid and lower alcohols. These methods are described, for example, in British Pat. No. 1,017,806, U.S. Pat. Nos. 3,639,460, 3,639,461, Japanese Patent Publication Nos. 13964/1968, 1369/1973, 42857/1973, Japanese Patent Laid Open Nos. 124018/1974, 45020/1974, etc.
The methods described in British Pat. No. 1,017,806 and U.S. Pat. No. 3,639,461 are directed to vapor-phase esterification procedures. These methods, however, are not preferred from a commercial standpoint in that there is a danger of pipe blockages due to polymerization of methacrylic acid esters at the high reaction temperatures.
The method described in U.S. Pat. No. 3,639,460 is a so-called methacrylic acid excess method in which the molar ratio of lower alcohol to methacrylic acid present in a reactor is less than 1. It is theoretically excellent. However, various difficulties are encountered in carrying out this method since a large amount of methacrylic acid is present in a reactor at high temperatures and concentrations. This easily leads to the formation of undesirable quantities of polymers.
The other methods are directed to liquid-phase reactions, in which an excess of a lower alcohol is used. The procedures described in Japanese Patent Publication Nos. 13964/1968, 1369/1973, Japanese Patent Laid Open No. 124018/1974 use a cation exchange resin as a catalyst. Methods using a cation exchange resin as a catalyst have the advantage that handling of the reaction mixture after esterification is simplified compares to those procedures in which sulfuric acid is used as a catalyst. They are not completely satisfactory, however, since the reaction rate is low in comparison with the sulfuric acid method, and the cation exchange resin deteriorates, resulting in a gradual reduction in catalytic capability.
In the method described in Japanese Patent Publication No. 13964/1968, for example, the unreacted methacrylic acid cannot be completely recovered. When methyl methacrylate is intended to be produced effectively by the above method using a cation exchange resin as a catalyst, it is not possible to increase the conversion of methacrylic acid unless a large excess of methanol is used. In addition, it is necessary to employ severe reaction conditions, e.g., to increase the reaction rate or lengthen the reaction period. The more rigorous reaction conditions cause serious loss of methacrylic acid due to polymerization.
In Japanese Patent Publication No. 1369/1973, a method is disclosed in which acrylic acid is esterified, but there is no example showing esterification of methacrylic acid. When the same method is applied to methacrylic acid, the reaction rate decreases. In order to compensate the reduction in reaction rate, it is necessary, as described above, to increase the reaction temperature or lengthen the reaction period. As previously mentioned, however, this causes loss of methacrylic acid due to polymerization.
This fact is clearly shown in an example of Japanese Patent Laid Open No. 124018/1974, in which the esterification is carried out in the presence of a hydrocarbon at a temperature of 110.degree. C. The temperature at which the reaction solution is separated is as high as 140.degree. C. The temperature at which the reaction solution is separated can be somewhat decreased by effecting the separation under reduced pressure. However, it is not desirable to increase the rate of esterification by raising the reaction temperature because of the resulting heat polymerization of methacrylic acid and methacrylic acid esters.
Because of these difficulties, sulfuric acid is the most commonly used esterification catalyst. Sulfuric acid has the advantage of low cost coupled with the ability of absorbing water formed during the esterification reaction, thereby increasing the rate of esterification.
Recently, however, the costs of alkalis required for neutralizing the waste sulfuric acid after esterification have increased. Moreover, water criteria for industrial waste water have been made more stringent. It is now necessary in many areas that the waste liquor from esterification reaction be subjected to additional, and costly, purification procedures.
Thus, although the sulfuric acid method using a large amount of sulfuric acid has certain advantages, it is now losing its advantages compared to other procedures, since the disposal processing of the waste sulfuric acid is highly expensive.